linux-kernel - Re: [PATCH v2 1/2] fork: free vmapped stacks in cache when cpus are offline

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Date:   Thu, 9 Feb 2017 09:38:25 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Hoeun Ryu <hoeun.ryu@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2 1/2] fork: free vmapped stacks in cache when cpus are
 offline

On Thu 09-02-17 13:03:46, Hoeun Ryu wrote:
>  Using virtually mapped stack, kernel stacks are allocated via vmalloc.
> In the current implementation, two stacks per cpu can be cached when
> tasks are freed and the cached stacks are used again in task duplications.
> but the cached stacks may remain unfreed even when cpu are offline.
>  By adding a cpu hotplug callback to free the cached stacks when a cpu
> goes offline, the pages of the cached stacks are not wasted.
> 
> Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
> ---
> Changes in v2:
>  remove cpuhp callback for `starup`, only `teardown` callback is installed.
> 
>  kernel/fork.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 61284d8..7911ed2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -170,6 +170,22 @@ void __weak arch_release_thread_stack(unsigned long *stack)
>  static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
>  #endif
>  
> +static int free_vm_stack_cache(unsigned int cpu)
> +{
> +	int i;
> +
> +	for (i = 0; i < NR_CACHED_STACKS; i++) {
> +		struct vm_struct *vm_stack = this_cpu_read(cached_stacks[i]);

the callbak will run on the given cpu so this_cpu_read will be in fact
per_cpu_ptr(cached_stacks[i], cpu). Using this_cpu_read is just too
confusing. Also you do want to make this function defined only for
CONFIG_VMAP_STACK.

> +		if (!vm_stack)
> +			continue;
> +
> +		vfree(vm_stack->addr);
> +		this_cpu_write(cached_stacks[i], NULL);
> +	}
> +
> +	return 0;
> +}
> +

-- 
Michal Hocko
SUSE Labs